This invention relates to construction of microchannel structures for use in microfluidic manipulations.
Microchannel structures are of great interest for applications involving the manipulation of small fluid volumes, such as chemical and biochemical analysis. Various microchannel structures having channel dimensions on the order of one or a few millimeters have been used for chemical and biochemical assays.
These structures are typically produced by injection molding using various thermoplastic polymers. Injection molding is an economical process, and a variety of thermoplastics having good optical and mechanical properties can be processed by injection molding to form the desired structures. The injection molding process involves introducing a molten thermoplastic material into a mold cavity, and then cooling the cavity to solidify the resin. In the case of forming microchannel structures, a mold having the negative pattern of the desired channel structures must be created. Conventional tooling methods can be used to create molds for channels having dimensions as small as about 1 mm. Typically, enclosed microchannels are desired for the final structure. A common method for enclosing microchannel structures formed in plastics is to join a base and cover substrate using sonic welding. In addition, certain adhesives can also be used to join the base and cover substrates.
It has become desirable to create microchannel structures having capillary dimensions, i.e., having dimensions ranging from less than 1 micron to upwards of 1 mm. These structures are of interest for manipulating very small fluid volumes through the application of electric fields to perform electrofluidics, i.e., the movement of fluids in microchannels utilizing electrokinetic flow, that is, electrophoresis and/or electroosmotic flow (EOF). Electrophoresis is the movement of individual charged particles or molecules in response to the application of an electric field to an ionic solution. Electroosmotic flow is a bulk fluid flow (individual ions plus solvent molecules) that also results from the application of an electric field to an ionic solution. The extent of the bulk fluid flow is a function of the charge on the wall of the channel, as well as the viscosity of the solution. Both EOF and electrophoresis can be used to transport substances from one point to another within the microchannel device.
To create microchannels having capillary dimensions, photolithography in silicon or glass substrates has been employed. See, e.g., U.S. Pat. No. 4,908,112, U.S. Pat. No. 5,250,263. In the case of fused silica, these structures can be enclosed by anodic bonding of a base and cover substrate.
Although microchannel structures of such materials have been produced, it would be much more economical, and therefore desirable, to produce structures of capillary dimensions in polymeric materials or plastics. However, the conventional methods for forming and enclosing channels in plastic do not provide the accuracy and precision required for structures of capillary dimensions. For example, when using sonic welding, heating and deformation may occur in the channel regions. When the edges of a sonic weld are uneven, poor electrofluidic performance may result. Furthermore, sonic welding of highly defined intersections of capillary dimensions is not easily accomplished with adequate fidelity. Similarly, with conventional adhesive methods, the adhesive material may flow into and plug the channels.
Thus, there is interest in the development of new methods of fabricating polymeric microstructures, specifically in new methods of sealing the cover and base plates together, where such new methods do not result in deformation or filling in of the microchannels enclosed in the structure. Ideally, such methods should be simple and readily reproducible so as to be suitable for large scale manufacturing.
U.S. Pat. No. 5,376,252 to Eckstrom et al. describes a process for creating capillary size channels in plastic using elastomeric spacing layers. Ohman International Patent Publication WO 94/29400 describes a method for producing microchannel structures involving the application of a thin layer of a thermoplastic material to one or both of the surfaces to be joined, then joining the surfaces and heating the joined parts to melt the thermoplastic bonding layer.